The genus Lolium is indigenous to Europe, the North Atlantic Islands, temperate Asia, and North Africa (Terrell, 1968). However, the distribution and evolution of the genus has been affected by the involvement of humans who have encouraged its spread, not only, as a crop (L. perenne L. and L. multiflorum Lamarck—selected for forage and amenity characteristics), but also has encouraged the spread and evolution of other members of this genus as weeds in primitive agriculture [(L. remotum Schrank associated as a weed with flax, (Linum usitatissimum L.); and L. temulentum L. as weed in wheat (Triticum aestivum L.)] (Jenkin 1936). These later two Lolium species are only known as weeds of cultivated crops and probably evolved in close association with primitive agriculture (Terrell 1968).
Essad (1954) recognized five species within the genus and separated the species into two distinct groups based on reproductive biology: 1) an allogamous (cross-pollinated and largely self-incompatible) group including L. perenne, L. multiflorum, and L. rigidum Gaudin; and 2) an autogamous (self-pollinated and self-compatible) group including L. temulentum and L. remotum. Terrell (1968) published a taxonomic revision of the genus, in which he maintained the same classification of Essad (1954), recognizing two reproductively different groups (allogamous and autogamous), but recognized two additional species in the allogamous group and one additional species in the autogamous group.
Terrell's classification is a follows: 1) the L. perenne group or allogamous (cross-pollinated and largely self-incompatible) group: L. perenne, L. multiflorum, L. rigidum Gaudin, L. subulatum Visiani, and L. canariense Steudel; and 2) the L. temulentum group or autogamous (self-pollinated and self-compatible) group: L. temulentum, L. remotum, and L. persicum Boissier & Hohenacker.
Aramendia (2005), in a revision of the genus Lolium, recognized the same eight as Terrell (1968), but also treated L. loweii Menezes (endemic to the Madeira Islands) as distinct from L. rigidum. Also, since Terrell (1968), two additional species of Lolium have been described: L. grandispicum Fei from China (Fei 1999) and L. edwardii Scholz, Stierstorfer & Gaisberg endemic to the Island of El Hierro in Canary Islands (Scholz et al. 2000). However, there is now a debate as to their taxonomic distinctness. Regarding L. grandispicum, this taxon it is now considered a synonym of ×Festulolium braunii (K. Richt.) A. Camus, an intergeneric hybrid [Festuca pratensis×Lolium multiflorum]. Lolium edwardii is sometimes treated as a synonym of L. canariense. Thus, Lolium currently consists of 9-10 species worldwide, with six (or seven) taxa in the allogamous group: L. perenne, L. multiflorum, L. rigidum, L. subulatum, and L. loweii, and L. canariense (and possibly L. edwardii); and three taxa in the autogamous group: L. temulentum, L. remotum, and L. persicum. 
Within five members of the L. perenne group (L. perenne, L. multiflorum, L. rigidum, L. subulatum, and L. loweii), there appears that repeated hybridizations and introgression has been occurring during the past several thousand years of human disturbance of habitats in the Mediterranean and southwest Asia.
Perennial ryegrass (Lolium perenne L.) is a cool-season perennial grass that is an important turf (amenities) and forage that has been introduced throughout the temperate world. In Britain, it is recorded as being deliberately sown and cultivated for pasture as early as 1677 (Plot, 1677, Beddows 1953). It is most extensively used in the United States and Europe as turf and forage. In Japan, Australia and New Zealand it used predominantly for forage, but is also important for golf courses and sports fields.
Turf-type (amenity) perennial ryegrass is used in temperate Europe, where it is naturalized, on winter games pitches and also for heavy-duty lawns, landscaping tennis courts, cricket fields, golf tees and fairways. It is sometimes used on its own, but often in mixture with red fescue (Festuca rubra L.) and bentgrasses (Agrostis spp.). In North America it is usually planted as a monostand with one or more varieties blended for lawns, or mixed with Kentucky Bluegrass (Poa pratensis L.) for sports fields and lawns. It is also used at lower latitudes to overseed winter dormant warm-season turfgrass areas. In New Zealand and Australia it is used extensively as both a permanent turf for sports pitches and racecourses, and for winter overseeding of warm-season turf areas (Thorogood 2003).
Terrell (1968, 2007) provides the following description of perennial ryegrass. Plants are a short to long-lived perennial, 3-100 cm tall, with culms that are erect, spreading, decumbent, or rarely prostrate, sometimes rooting at the lowest nodes, slender, usually with 2-4 nodes aerial below the spike, glabrous or scaberulous just below the spike. Basal leaf sheaths green, reddish, purplish or in age straw colored, sometimes papery in texture, glabrous; upper leaf sheaths green, glabrous. Leaf blades folded (not rolled) in bud of young shoots; mature blades usually 10-30 cm long, 1-6 mm wide, with ca. 20 veins visible on adaxial surface, glabrous, and abaxial surface of blades are shiny, smooth, glabrous; margins glabrous to scaberulous. Ligules to 2.5 mm long, membranous, apex rounded to truncate or erose, glabrous. Auricles present to 3 mm long, or absent. Spikes are 3-31 cm long, straight or slightly curved, usually ¼ to ½ the height of the plant, bearing 5-37 spikelets. Rachis (or central axis of the inflorescence) is slender, often flexuous, 0.6-2.5 mm in width at nodes; internodes concavo-convex or concavo-angular in cross section, glabrous or scaberulous on angles, with the spikelets lying against the concavities of the rachis. Spikelets are 5-22 mm long, 1-7 mm wide, with 2-10 fertile florets and 0-1 sterile rudimentary florets distally, rachilla 0.7-2 mm long, somewhat flattened; glumes, the lower (first) glume is absent, except in the terminal spikelets; upper (second) glumes 3.5-15 mm long, 0.7-1.5 mm wide, with 3- to 9-veins, ⅓-¾ as long as to slightly exceeding the distal florets and somewhat longer to somewhat shorter than the lowest floret, membranous to indurate; lemmas, of lower and middle florets, 3.5-9 mm long, 0.8-2 mm wide (about 4 to 8 times longer than wide), 3- to 5-veined, glabrous or glabrate, oblong or ovate in shape, rounded on back, the hyaline apices are obtuse, acute, slightly bifid, or erose; awns usually absent, when present, to 8 mm long, attached 0.2-0.7 mm below the apices; paleas shorter than (to 1 mm shorter) to slightly longer than the lemmas, apices acute or obtuse, keels with minute teeth; anthers 2-4.2 mm long, 0.3-0.7 mm wide, yellow or purplish. Caryopses 3-5.5 mm long, 0.7-1.5 mm wide, 3 or more times longer than wide. Chromosome Number 2n=14 or 28 (in some commercial cultivars). However, the production of stolons by perennial ryegrass was not reported by Terrell (1968, 2007). Terrell (1968) commented was that the culms are, “rarely prostrate and sometimes rooting at the lowest nodes.”
Stolons are horizontal, aboveground stems that root at the nodes and can produce new plants from their nodes. For example, white clover (Trifolium repens L.), strawberries (Fragaria×ananassa Duchesne), creeping bentgrass (Agrostis stolonifera L.), and bermudagrass [Cynodon dactylon (L.) Persoon] propagate themselves with stolons. The stoloniferous habit of perennial ryegrass can also sometimes be found in grazed pastures. There are a number of references reporting stoloniferous ryegrass in grazed pastures. The stoloniferous habit in perennial ryegrass was first reported by Sinclair (1826). Lawson (1836) in his work discussing the plants cultivated, or capable of being advantageously cultivated in Great Britain for herbage and forage, listed 10 of the most important varieties of Lolium perenne for cultivation for livestock. In his list, he discusses the discovery of a remarkably stoloniferous ryegrass collected from Germany. He found this collection so unique that he described it as a new variety: L. perenne var. stoloniferum. This stoloniferous variety was described as follows: “It is of early spring growth, pushing out long prostrate stolons or shoots, with an abundance of foliage, so that one plant, by the time the spikes begin to appear, will form a close tuft, extending from two to three feet in diameter; the shoots, however; although lying on the ground, never attempted to strike root until near the end of the season, and even then very sparingly.”
More recent publications report the occurrence of a ‘stoloniferous’ or ‘rhizomatous’ habit in perennial ryegrass that actually is a pseudo-stoloniferous or pseudo-rhizomatous habit and are not true stolons or rhizomes. From observations of perennial ryegrass in New Zealand pastures, Mitchell (1956) stated that “the vegetative shoots of the plant can, where trampled into the soil and buried, adopt a rhizomatous [pseudo-rhizomatous] habit of growth.” Kydd (1966) described the spatial orientation of ryegrass tillers under two stocking rate treatments in Hurley, England. Repeated grazing to ground level induced the formation of horizontal tillers, and in the paddocks grazed hard until late spring and then closed for silage these tillers showed stem elongation and produced buds and adventitious roots at the nodes. Kydd (op. cit.) described these as “true stolons.” Edmund (1964) suggested stolon formation would be encouraged under grazing, in ryegrass genotypes that had the ability to elevate nodes, and hence potential tiller sites, from the buried crown and this is probably a factor enabling perennial ryegrass to withstand heavy treading. Further observations on stoloniferous ryegrass in Northern Ireland paddocks were made by Hayes (1971). Simons et al. (1974) demonstrated that aerial tillering was encouraged by an increased height of cutting and straw mulch, and that it differed in extent according to genotype. Harris et al. (1979) reported finding an underground stoloniferous (i.e., pseudo-rhizomatous) growth habit in old turf where crowns had been buried with soil and thatch. They found dead crowns 3-4 cm below the soil surface and the dead crowns were connected to what they termed ‘underground stolons.’ These ‘underground stolons’ were reported to be fragile with only the culm itself remaining, with leaves having decomposed away. This was similar to what Mitchell (1956) reported finding in livestock pastures.
In the above cases, these “stolons” are actually: 1) aerial culms that get trampled or pushed down, and then begin to root a the nodes; or 2) plant crowns that become buried by soil, dung, mulch, earthworm casts, etc., and the with the leaves dying and decomposing leaving only the culm, and rooting at nodes. But, in these instances these cannot be considered true stolons in the technical sense, but would be classified as pseudo-stolons or a pseudo-stoloniferous habit in the first instance and has pseudo-rhizomes in the second instance, since the culms were buried.
Oakley and Evans (1921) categorized the stolons (and rhizomes) of timothy (Phleum pratense L.) into two types: determinate- and indeterminate-stolons. A determinate-stolon is an above-ground horizontal stem which roots at the nodes and does not produce aerial shoots indeterminately, but the apical meristem will eventually terminate with an inflorescence (e.g., referred to herein as Lolium perenne subsp. stoloniferum). An indeterminate-stolon is an above ground stem which roots at the node and from which shoots are produced progressively and this horizontal stem will never terminate with an inflorescence, but apical meristem remains vegetative (e.g., bermudagrass and creeping bentgrass).
Perennial ryegrass is an important species for sports fields. Though perennial ryegrass is one of the most wear tolerant cool-season (temperate) turfgrasses available, the demand for more wear tolerance has increased to due to increased use of sports fields, parks, golf courses, and recreational areas. Improvements in summer wear tolerance have been achieved previously indirectly by increasing shoot density (Thorogood, 2003). Winter wear on European sports pitches has been reduced partly by empirical evaluation of wear-resistant of ryegrass varieties using artificial wear machines with studded rollers and using those varieties most wear-resistant (Canaway, 1981). These were only evaluations performed on finished varieties to determine if some may have some wear tolerance. However, no selections were performed and no new wear-resistant varieties were developed from these studies. Traffic simulation is mainly performed to evaluate the wear-resistant of already released cultivars (e.g., Canaway 1981; Cockerham 1989; Anon 2001) or for athletic field research. The goal in using traffic simulation in athletic field research is to subject turfgrass areas to the conditions experienced by actual playing surfaces (Henderson et al. 2005). This research is used to contribute information on the effects of traffic stress on turfgrasses or playing surfaces (Vanini et al. 2007), not on the development of genetically superior populations of wear-resistant turfgrasses.
The amount of biomass produced before wear commences has been found to be a good indicator of winter wear tolerance (Ellis, 1981). Such material will have an equivalent higher biomass after wear and will also maintain a greater leaf area index with a greater photosynthetic capacity for recovery regrowth. This will be more important in winter when light and temperature may well limit photosynthetic activity.
Lush and Rogers (1992) found that turf follows the general ecological principle of the self-thinning rule, where biomass is inversely proportional to shoot density (White, 1981). This rule states, for a given turf population, there is a ceiling (self-thinning line) where biomass can only be increased with a reduction in shoot density. For example, lowering the cutting height will increase shoot density, while raising the cutting height will increase biomass. The correlation between shoot density and biomass in ryegrass cultivars is weak (Shildrick, 1981), and hence many cultivars listed by the STRI (Sports Turf Research Institute, Bingley, West Yorkshire, UK; which is the independent market leader in consultancy and research for sports surfaces) as having good wear tolerance have poor shoot density and visual appeal (Anon. 2001). The lack of association between wear tolerance and visual appeal (shoot density) was also reported by Bonos et al. (2001).
Currently, little is known or published, about the mechanisms and genetic control of resistance and tolerance on which more directed selection could be practiced (Thorogood 2003). But the inventors have discovered through a novel intense selection protocol involving artificial traffic simulation in the actually breeding process, which not only has improved the wear tolerance and visual appeal of these perennial ryegrasses, but also, unexpectedly, selected for ryegrass genotypes with determinate-stolons that enhance the wear tolerance and have an increased regenerative potential to recover from the traffic event.